In cone crushers there is a perpendicular eccentric shaft and in that shaft an oblique inner bore. In the bore is fitted a main shaft, to which shaft a crushing head is usually attached. The head is laterally surrounded by the crusher frame, to which frame is attached a concave, functioning as a wear part. To the head is correspondingly attached a mantle, functioning as a wear part. The mantle and the concave together form a crushing cavity, and within this, feed material is crushed. When the eccentric shaft is rotated, the main shaft together with the head is forced into an oscillating movement, whereby the gap between the mantle and the concave varies at each location during the work cycle. The smallest gap during a work cycle is called the crusher setting, and the difference between maximum gap and minimum gap is called the crusher stroke. By means of the crusher setting and the crusher stroke, e.g. the particle size distribution of produced crushed stone and the production capacity of the crusher can be controlled.
The main shaft of a crusher is often supported in the crusher frame by a top bearing at its upper end. This subtype of cone crusher is usually referred to as a gyratory crusher.
A gyratory crusher is usually adjustable by a hydraulic system thus, that the main shaft can be moved vertically relative to the crusher frame. This makes possible a change of the setting so, that the particle size of crushed stone will conform to the required size, and/or keeping the setting constant during wearing of the wear parts.
In other types of cone crushers, the adjustment can be made also by raising and lowering the upper crusher frame and the concave attached to it relative to the lower crusher frame and to the main shaft, which remains vertically stationary relative to the lower frame.
In a crusher there are many surfaces associated by sliding bearings. Depending on the type of crusher these include, for example, surfaces between:                main shaft and eccentric shaft        eccentric shaft and lower frame        eccentric shaft and adjusting piston        main shaft and upper frame        main shaft and adjusting piston        
The above-mentioned members are not usually in immediate contact with each other, but in practice there are usually one or more bearing sleeves between them, so the actual bearing surfaces usually form between the above-mentioned members and these bearings.
When the bearings of a crusher work properly, friction forces between the crusher bearing surfaces are minimal. If a crusher is affected by a disturbance in lubrication, the friction forces between bearing surfaces will increase and the bearings will be in danger of seizure. This kind of disturbances can be, for example, crusher overload, contaminants in lubricating oil, or pressure or flow decrease of lubricating oil.
Seizure damage has a tendency to advance in a crusher from one bearing surface to another. Lets assume, for example, that there is a disturbance in the lubrication between the main shaft and the eccentric shaft of a crusher and the mentioned bearing surfaces start to seize. This causes heating of the eccentric shaft. When the heat is conducted to the bearing surface between the eccentric shaft and the crusher frame, the lubrication of this bearing surface can also be impaired, which causes also this bearing surface to start to seize.
The seizure described in this example can also advance in the opposite direction from one bearing surface to another, or it can also advance between other bearing surfaces.
The case described as an example can lead also to a situation, where the bearings of a crusher along with the main parts of a crusher, such as the frame, the main shaft, the eccentric shaft etc. are completely damaged. Repair costs of this kind of total damage are difficult to estimate, because the costs differ greatly from case to case depending on, for example, the crusher type. On an average, the costs may be about between EUR 20 000-50 000 (at year 2000 prices). In addition, the stoppage of a crusher causes considerable costs.
In Finnish patent 100554, a method is disclosed for monitoring the condition of crusher bearings by monitoring the rotation speed of a main shaft around its axis. When the gap of a crusher is full of stones, these stones will decrease the rotation speed of the head and the main shaft. Thus, a relatively large increase in friction between the main shaft and the eccentric shaft is necessary before a change in the rotation speed of the main shaft can be detected. At this point, the damage at the sliding surface between the eccentric shaft and the main shaft has already advanced relatively far.